Paper and oil insulated high voltage power cables rely on maintaining the insulating oil under pressure for the proper functioning. In the case of the so-called Pipe Type Cable (three insulated conductors pulled into a common steel pipe) the pressure is normally maintained by a pump drawing oil from an oil tank. The so-called Self Contained Cable is normally kept under pressure by means of static oil reservoirs, although in certain cases a "Pumping Plant" may also be used for this type of cable, particularly in the case of long submarine crossings.
One reason why pumping plants are being used also for Self Contained Submarine Cables is that with this system it is reasonably inexpensive to store a large quantity of oil in case the cable should become ruptured, so that one might prevent water from entering the cable by creating a certain small outflow of oil at the rupture point. The pumping plant is, however, dependent on supply of electricity. The oil reservoir system does not require electricity, but the cost of providing extra oil capacity for a possible cable rupture becomes prohibitive.
For pumping plants, pumps relying on means other than electric motors are known to have been used as a back-up in case of power failure. The back-up pump may have been driven by an air motor, fed either with air or nitrogen stored under high pressure in gas cylinders.
If a suitable system could be developed which could act as a back-up for a pumping plant, such a system ought to be suitable also as a back-up for pressure reservoirs in case a leak should develop in the cable system.
Unfortunately the oil in a Self Contained Cable must be virtually completely free from moisture and gas in order to function properly as an insulation together with the paper. For this reason it is, for instance, not permissible to store such oil under a "blanket" of nitrogen, such as is normally the case for a pipe-type cable pumping plant. For this reason it is known to use so-called "canned" motors to drive the pump in pumping plants for this type of cable. Use of these hermetic motors excludes the need for a rotating seal on the shaft between the motor and the pump. Such a shaftseal is particularly undesirable in the case of a pump which is to stand as a back-up, maybe for several years, before it is called upon to pump oil.
Hermetic motor-pump combinations for air driven motors without a rotating seal do not exist. The pumps which come closest to being "hermetic" are those using the diaphragm or membrane principle. One such pump (marketed by THE WARREN RUPP CO. Mansfield, Ohio under the trade name "Sand Piper" Pump Model No. SAI-A or SB1-A) utilizing two membranes connected by a rod and with a driving gas in contact with the "rod-side" of the two membranes, seems particularly well suited for the purpose. A diaphragm or membrane pump of this type is shown in Operating Instructions, Service Manual and Repair Parts List, issued as Form No. SPL-2/77R by The Warren Rupp Co. and fully disclosed in U.S. Pat. No. 3,782,863, whose disclosure is incorporated herein by reference. The only disadvantage is that it utilizes a membrane which is not made of metal, and therefore, in time, will allow gas and moisture to permeate through the membrane to the cable oil on the other side.